As part of our investigations to understand why the Cam is so murky (technically known as “turbid”) we are monitoring the turbidity at Sheeps Green, roughly every month.
As the Cam is quite deep here, we do this using a “Secchi disk”, measuring the depth at which the black and white segments are no long distinguishable. This is called the “Depth to Invisiblity” or DTI.
The results are shown on the chart here. The greater the “Depth to Invisibility” (DTI) the clearer the water.
We know that the turbidity is mostly due to suspended soil and fine clays. All readings are taken when the Cam is flowing gently (ie not in flood) so we hope that the results may help indicate whether the primary cause is agricultural or the burrowing of the invasive American Signal Crayfish.
American Signal Crayfish are most active in the summer, but inactive in their burrows during the winter. If they were a primary cause of the turbidity, one might expect it to be worse in the summer (ie DTI readings to be lower)
If agriculture was the problem, it might be expected to be worse in the winter, when there was less vegetation on fields to retain the soil, or following any upstream dredging work.